Forests are often described as the lungs of the world, but their climate role is more precisely understood as a vast biological carbon-storage system. Through photosynthesis, trees remove carbon dioxide from the atmosphere and convert it into organic matter, including trunks, branches, bark and leaves. When forests accumulate more carbon through growth than they release through death, decay, fire or disturbance, they function as carbon sinks. The new research focuses on this above-ground component, which represents a major share of the carbon stored in living forest vegetation and provides a direct measure of how forest ecosystems are changing over time.

The study by V.A. Maia, N. de Aguiar-Campos, F. Coelho de Souza and colleagues is significant because it examines the trajectory of forests over the long term rather than treating carbon uptake as a fixed property. Tropical forests have absorbed a substantial portion of human carbon emissions in recent decades, helping to moderate the pace of atmospheric warming. Yet that service depends on a delicate balance between tree growth and carbon losses. If growth slows, mortality increases or disturbances become more frequent, the sink can weaken even when the forest still appears green from above.

Brazil contains an extraordinary range of forest ecosystems, from the humid Amazon rainforest to subtropical forests in the south and transitional landscapes shaped by seasonal rainfall. These ecosystems differ in climate, soil, species composition and disturbance history, but they are connected by the same basic carbon cycle. Trees take in carbon dioxide, move it into wood and tissue, and eventually return it to the atmosphere through respiration and decomposition. A forest can therefore remain standing while its net carbon uptake declines, an important distinction that satellite images alone may not reveal.

The researchers’ finding points to a shift in that balance. A declining above-ground carbon sink means that the forests are still storing carbon, but their net accumulation is weakening over time. This does not necessarily mean that every forest site is losing carbon or that all Brazilian forests are responding identically. Instead, the result describes a broad directional change across tropical and subtropical forest systems. Such a pattern is especially important because the atmosphere responds to the combined carbon balance of landscapes, not simply to whether individual trees remain alive.

Several forces may be contributing to the decline. Rising temperatures can increase the physiological stress experienced by trees, particularly when heat coincides with drought. Water shortages can close the microscopic pores in leaves, reducing photosynthesis and limiting growth. At the same time, warmer conditions can raise respiration rates, increasing the amount of carbon plants release while maintaining their tissues. Severe drought can also trigger hydraulic failure, in which trees are unable to transport water from roots to leaves, or carbon starvation, in which prolonged stress leaves them without enough energy to sustain vital functions.

Disturbance adds another layer of pressure. Deforestation directly removes biomass, while selective logging, fires, storms and fragmentation can damage forests without eliminating every tree. Forest edges are often hotter, drier and more exposed to wind than intact interiors, creating conditions that can increase mortality and reduce regeneration. Fire is particularly consequential because it rapidly transfers stored carbon into the atmosphere and can alter soils, vegetation structure and the likelihood of future burning. Even areas that eventually recover their canopy may take decades to rebuild the carbon held in mature trunks and large branches.

The study also challenges a comforting assumption in climate policy: that natural carbon sinks will continue absorbing carbon at historical rates while societies reduce emissions. Forest sinks are not machines operating at a constant capacity. Their performance depends on climate, ecological interactions and the history of disturbance. As atmospheric carbon dioxide rises, some trees may initially grow faster, but that fertilization effect can be constrained by nutrients, water availability, temperature and competition. A forest cannot convert unlimited carbon dioxide into biomass if other ingredients required for growth are missing.

This matters far beyond Brazil. The carbon absorbed by forests is included in many climate projections and national emissions strategies, yet the future strength of those sinks remains uncertain. If tropical forests absorb less carbon than expected, the atmosphere could accumulate carbon dioxide more rapidly than models and policy plans anticipate. That would increase the amount of emissions that must be avoided through energy, transport, industry and land-use reforms to achieve the same climate targets. It also means that protecting forests is not only a biodiversity priority; it is a way of preserving a climate service whose value may be declining under pressure.

The findings are especially relevant to Brazil’s efforts to curb deforestation and restore degraded land. Preventing the loss of mature forests protects existing carbon stocks, but conservation alone may not fully restore the forests’ former capacity to absorb additional carbon if warming and drought continue. Restoration can rebuild biomass, improve habitat connectivity and strengthen ecological resilience, but newly planted or regenerating forests do not immediately replace the carbon-storage function of old-growth ecosystems. Effective strategies will therefore need to combine protection, restoration, fire prevention and climate adaptation while addressing the emissions that intensify heat and hydrological stress.

Measuring these changes is technically difficult. Above-ground carbon is not observed directly across every hectare; it is estimated using combinations of field inventories, tree measurements, biomass equations, remote sensing and ecosystem models. Researchers typically convert tree dimensions, such as trunk diameter and height, into estimates of biomass and then into carbon content using established relationships. Repeated measurements reveal whether a forest parcel is gaining or losing carbon, while satellite observations help extend information across much larger areas. Each method carries uncertainty, but agreement across long-term observations can expose trends that short studies might miss.

The importance of a long-term perspective is difficult to overstate. Forest carbon dynamics can fluctuate from year to year because of rainfall, El Niño events, fires, storms and localized outbreaks of pests or disease. A single unusually productive season may suggest that a forest is recovering, while a short period of mortality may exaggerate the appearance of permanent decline. Long records allow researchers to distinguish temporary variation from a sustained change in the underlying carbon balance. They also help identify whether declines are concentrated in particular climates, forest types or regions, information that is essential for designing targeted conservation measures.

The message from Brazil’s forests is therefore both urgent and nuanced. These ecosystems remain indispensable carbon reservoirs, but their ability to keep removing additional carbon from the atmosphere cannot be treated as guaranteed. The reported decline does not erase the value of forests; it makes their protection more consequential. Every avoided clearing, prevented fire and preserved mature tree represents carbon that remains locked away, as well as habitat, rainfall regulation and protection for countless species.

As climate change intensifies, the future of the forest carbon sink will depend on decisions made both inside and outside the forest. Brazil’s landscapes will be shaped by land-use enforcement, Indigenous and local stewardship, restoration, fire management and the pace at which the world cuts fossil-fuel emissions. The new study’s central warning is straightforward: tropical and subtropical forests are still helping humanity, but they may be helping less than before. In a warming world, that fading service could become one of the clearest signals that ecological limits are arriving faster than expected.

Subject of Research: Long-term changes in above-ground carbon storage and carbon uptake in Brazilian tropical and subtropical forests

Article Title: Long-term decline in above-ground carbon sinks in Brazilian tropical and subtropical forests

Article References: Maia, V.A., de Aguiar-Campos, N., Coelho de Souza, F. et al. “Long-term decline in above-ground carbon sinks in Brazilian tropical and subtropical forests.” Nature Communications (2026). https://doi.org/10.1038/s41467-026-74921-0

Image Credits: AI Generated

DOI: 10.1038/s41467-026-74921-0

Keywords: Brazilian forests, tropical forests, subtropical forests, carbon sinks, above-ground biomass, climate change, forest carbon storage, deforestation, drought, carbon cycle

Forests act as global carbon sinks, removing vast amounts of atmospheric carbon dioxide and mitigating climate change effects. However, prolonged and increasingly severe drought conditions challenge this role, weakening tree vitality and increasing mortality rates. The study, led by researchers Gazol, Pizarro, Hammond, and colleagues, systematically analyzed decades of climate and remote sensing data, alongside extensive field observations, to dissect the temporal patterns of drought preceding tree deaths. Their robust methodology allowed identification of a clear escalation in both the duration and intensity of droughts before mortality events, revealing an insidious trend that could foreshadow widespread forest decline.

The research team focused particularly on dry biomes — including Mediterranean, semi-arid, and arid zones — where water availability already limits vegetation growth. Within these ecosystems, droughts are natural events; however, the increasing span and severity of these dry periods challenge tree survival beyond historical norms. The study reveals that trees in these environments experience extended water deficit stress, which can exceed physiological thresholds critical for maintaining leaf function, photosynthetic activity, and hydraulic conductivity. As a consequence, trees weaken and become more vulnerable to secondary stressors, such as pest outbreaks and wildfires.

This intensification of drought stress is linked directly to global climate change mechanisms. Rising surface temperatures elevate evapotranspiration rates, reducing soil moisture retention and amplifying water stress. Simultaneously, altered atmospheric circulation patterns influence precipitation regimes, often decreasing the frequency but increasing the severity of rainfall events. These synergistic effects produce longer intervals between replenishing rainfalls, compounding water deficits and pushing trees into prolonged drought states before mortality occurs.

Previous studies have acknowledged that drought is a critical driver of tree mortality, but this new work precisely quantifies the shifting temporal characteristics of drought episodes. By analyzing a comprehensive dataset spanning several biomes and decades, the authors illustrate that the drought durations preceding mortality have increased substantially beyond natural variability. More strikingly, the magnitude of soil moisture deficits and vapor pressure deficits — indicators of atmospheric dryness and plant water stress — have intensified, setting a harsher stage for tree survival.

The consequences of prolonged pre-mortality drought extend beyond individual tree fates. Forests function as complex, interconnected systems where the loss of key individuals can alter species composition, reduce biodiversity, and weaken ecosystem resilience. In dry biomes, where trees often anchor fragile soil and regulate microclimates, increased mortality risks the fragmentation of habitats and the erosion of ecological niches. Disruptions at this scale may reduce carbon sequestration capacity, amplifying climate feedback loops and accelerating environmental degradation.

Highlighting geographic patterns, the study found that Mediterranean forests and woodlands are particularly vulnerable, exhibiting some of the most dramatic increases in pre-mortality drought intensity and duration. This aligns with observational evidence of recent widespread diebacks and raises urgent concerns about the future of these iconic and economically important ecosystems. The findings underscore a pressing need for adaptive forest management strategies tailored to anticipated climate trajectories — approaches that integrate drought risk into conservation planning.

To deepen understanding of physiological responses underpinning tree mortality, the researchers incorporated ecohydrological models evaluating how water transport limitations evolve during extended droughts. These models revealed that prolonged soil moisture deficits strain xylem function, the vascular tissue responsible for water conduction, often culminating in hydraulic failure. Once critical thresholds are crossed, trees lose the ability to maintain water supply to leaves, leading to desiccation and eventual death. This mechanism, referred to as ‘hydraulic failure,’ is now recognized as a primary cause of drought-induced tree mortality.

The research team also points out that longer droughts may not only weaken individual trees but also induce shifts in species composition over time. Drought-sensitive species may decline, while drought-tolerant species possibly gain competitive advantage, altering forest structure and function. Such compositional changes could influence carbon dynamics and increase susceptibility to invasive species or pest outbreaks, further challenging ecosystem stability.

Importantly, the investigation addresses how interactions between drought and warming temperatures exacerbate stress. Rising temperatures increase vapor pressure deficit (VPD), which intensifies evaporative demand on trees even when soil moisture is limiting. Elevated VPD can accelerate dehydration processes in leaves and impair stomatal regulation, compounding water stress during drought. This dual impact underscores the multifaceted challenges imposed by climate warming on tree survival.

Moreover, the study emphasizes that drought effects are not uniform across all forest types. While dry biomes show the most pronounced trends, the authors detected more subtle but still concerning patterns in mesic forests, indicating potential widespread vulnerability. This geographic gradient highlights the need for nuanced approaches toward forest conservation and restoration efforts, acknowledging biome-specific responses to climate stressors.

To obtain these insights, the researchers leveraged cutting-edge remote sensing technologies, combining satellite-derived soil moisture indices, thermal imaging, and canopy health measures with ground truthing campaigns. This integrative approach allowed for high-resolution monitoring of drought dynamics and mortality events at regional and global scales. Their multi-decadal dataset spans several continents, providing an unprecedented synthesis of drought-mortality relationships.

The implications of these findings ripple into policy spheres. Forest management practices must now contend with an evolving climate context characterized by amplified and prolonged drought conditions. Strategies such as assisted migration, selective thinning to reduce competition for water, and restoration of diverse species assemblages may prove pivotal in building drought resilience. Additionally, fire management becomes crucial, as dry forests with weakened trees show increased susceptibility to catastrophic wildfires that further degrade forest health.

From a climate mitigation standpoint, sustaining resilient forests amid increasing drought stress is vital to preserve their carbon sequestration functions. The study’s results urge integration of drought-induced mortality trends into global climate models, ensuring more accurate projections of carbon cycling feedbacks. Recognizing the escalating vulnerability of dry biomes is essential for informing international commitments aimed at climate stabilization and biodiversity conservation.

In conclusion, the study by Gazol, Pizarro, Hammond, and their collaborators presents compelling evidence that droughts preceding tree mortality events have grown not only longer in duration but also more intense—changes particularly acute in dry biomes experiencing heightened water scarcity. This intensification of drought stress challenges forest stability worldwide, with cascading repercussions for biodiversity, carbon dynamics, and ecosystem services. Moving forward, the scientific community, land managers, and policymakers must coordinate efforts to anticipate and mitigate these emerging threats, ensuring that forests remain robust allies in the global fight against climate change.

Article Title: Droughts preceding tree mortality events have increased in duration and intensity, especially in dry biomes.

Article References:
Gazol, A., Pizarro, M., Hammond, W.M. et al. Droughts preceding tree mortality events have increased in duration and intensity, especially in dry biomes. Nat Commun 16, 5779 (2025). https://doi.org/10.1038/s41467-025-60856-5

Image Credits: AI Generated

Recent research conducted by a team of international scientists reveals that the understorey is not merely a mundane backdrop but a vibrant component of the forest ecosystem that contributes substantially to woody biomass and biodiversity. Lead author Hermane Diesse, a doctoral candidate at the Namibia University of Science and Technology, elucidates that in drought-prone regions, such as Namibia, small trees and shrubs may account for nearly one-third of the total woody biomass. This pivotal finding underscores a critical oversight in current biomass assessments, which often disregard these smaller yet essential flora, thereby underestimating the ecological value of the Miombo woodlands.

The team focused on the nuanced interplay between the understorey vegetation and forest structure across varying climatic conditions. Their comprehensive analysis emphasizes how species diversity within the understorey shifts in response to the biomass density of the overstorey trees. Notably, as the overstorey biomass increases, the species richness of shrubs diminishes, revealing a fascinating dynamic whereby the presence of large trees can dictate the ecological narrative of the understorey.

Conversely, the research also illuminates that an increase in the biomass of larger trees does not inherently diminish the number of understorey tree species. Instead, dense overstorey canopies often serve as a conducive environment for the proliferation of young trees in the understorey, promoting a delicate balance within the ecosystem. This correlation indicates the intricate relationships that govern forest dynamics and challenges the simplistic views often held about these ecosystems.

Publishing their findings in the esteemed Forest Ecosystem Journal, the researchers initiated a broader discussion about carbon assessment protocols that predominantly focus on large trees while neglecting smaller woody plants. The traditional methodologies employed in carbon assessments have largely been modeled on findings from well-watered Miombo systems. This reliance poses significant gaps in understanding the comprehensive carbon stock estimates, particularly in arid and semi-arid ecosystems where the contribution of small trees and shrubs might be particularly significant.

One key revelation from this study centers around the notion that understorey biomass is resilient, remaining relatively unchanged even as one moves from dry to wet Miombo environments. This challenges the long-held belief that life in drier ecosystems is uniformly less productive, suggesting a complex vertical structure within Miombo woodlands that allows for diverse life forms to coexist and thrive amidst prevailing climatic constraints.

This research opens avenues for further investigations into the carbon sequestration capabilities of dry woodlands, which are often overlooked in broader sustainability dialogues. The understorey presents a remarkable potential for carbon capture, particularly in regions experiencing rapid changes due to climate variability. To optimize forest carbon stock predictions, it is essential to advocate for methods that account for the full spectrum of forest structure, including both the significant overstorey trees and the unknown riches hidden beneath.

Ultimately, Hermane Diesse stresses the urgency of recognizing and incorporating the complex dynamics of these ecosystems into conservation strategies. As the climate crisis escalates, it becomes increasingly vital to foster a holistic understanding of how various plant layers within ecosystems interact and contribute towards climate mitigation efforts. Ignoring the intricate contributions of understorey species would not only undermine scientific understanding but also impede the development of effective conservation policies.

This groundbreaking research stands as an essential pivot in the current environmental discourse, urging scientists, policymakers, and conservationists alike to broaden their perspective on biodiversity, especially in arid and semi-arid contexts. By illuminating the hidden treasures and the roles they play in sustaining ecological health, it beckons a shift towards more inclusive ecosystems management approaches that champion the diversity of life at all levels.

The implications of these findings are profound, suggesting that enhanced focus on understorey trees and shrubs might be necessary for accurately assessing the carbon emissions and potential sequestration in forested areas. As assessments evolve, so too must the approaches we take toward conservation and ecological restoration, which must now include a broader range of species within policy frameworks.

In summary, this pivotal research highlights a significant gap in the existing literature and reflects the need for a paradigm shift regarding how we assess carbon storage capabilities in forest ecosystems. As the world grapples with the realities of climate change, understanding and integrating the roles of these vital yet overlooked components could prove critical not only in forest conservation efforts but also in our global approach to carbon neutrality and biodiversity preservation.

Subject of Research: Small trees and shrubs in the understorey of Miombo ecoregion
Article Title: Quantifying Unseen Woody Biomass and Diversity in Understorey Trees and Shrubs at the Extremes of Water Availability in the Miombo Ecoregion.
News Publication Date: October 2023
Web References: http://dx.doi.org/10.1016/j.fecs.2025.100302
References: None available
Image Credits: Hermane Diesse, et al

Keywords: Miombo ecoregion, biodiversity, carbon storage, understorey vegetation, ecology, woody biomass, conservation, climate change, plant communities, forest management.